In the treatment of sleep apnoea and other respiratory disorders, a positive air pressure is used applied to the patient airway. The equipment used is known as an airflow generator.
A method used to generate air pressure is shown in FIG. 1. A brushless DC motor (16) is used to drive a turbine or blower (15). The turbine (15) generates the air flow for the patient. The brushless DC motor controller (14), in conjunction with the control electronics of the flow generator (13), receive power from a power supply (12) that is connected to the AC main through a filter (11). Sometimes the filter is built into the power supply itself. Control signals are sent from the control electronics (13) to the brushless DC motor controller (14), so the speed of the motor (16) can be controlled.
The pressure and the amount of air delivered depend on the speed of the turbine. In some types of equipment, pressure and flow sensors are used to monitor these variables and change the speed of the motor to achieve the desired effect. Also, in some cases, the speed of the motor is changed, alternating between a high and a low value, either in response to the patient respiration or as part of an automatic cycle. Equipment performing in this way is known as bi-level devices.
In FIG. 1, a single power supply (12) provides power to both, the motor driving circuits (14) and the control electronics (13).
A brushless DC motor, or BLDCM, is a DC motor with an electronic commutator. FIG. 2 shows a block diagram of one type of permanent magnet brushless DC motor along with its electronic commutator.
The driving electronics consist of a logic circuit (22) that controls a set of electronic switches (21) that switch power to the motor windings (23) much as the brushes do in a conventional DC motor. Current through the windings (23) generates forces in the rotating magnets (24), causing the rotor of the motor to spin. The switches (21) can connect the end of its corresponding winding to either the positive or the negative side of the DC voltage source, and also they can leave the winding unconnected.
The logic circuit (22) of the electronic commutator has as an output two control signals per switch, shown in the figure as signal groups SWC1 to SWC3, of two lines each. The motor has hall-effect sensors (25, 26 and 27) that are used by the logic circuit (22) to detect the position of the rotor and to switch the right waveforms to the windings (23). Typically, the industry uses a three phase motor (three windings) that is depicted in Y-configuration, for example, but may also be in a triangle configuration.
As the axis of the motor rotates, the motor windings are driven with three trapezoidal 6-step waveforms. During each step, voltage is applied to two windings only.
There is also a sensor-less mode of operation, in which a special controller monitors the voltage in the winding that has been left open-circuit to read the back-emf generated in the winding as the motor axis rotates.
In a CPAP application, like the one shown in FIG. 1, the BLDCM (16) takes considerable power especially during its acceleration periods. In a typical CPAP application, a motor can take around two amps at 24 volts (or more if a 12 volt motor is used), depending on the pressure and flow generated, and the particular motor chosen. The electronics necessary to perform the control, however, can be designed so the electronics take under 0.1 amps of current at a relatively low voltage. Most of the electronics can work with 5 volts, while only the pressure and flow sensors may need more, depending on the implementation.
The power supply for the motor (16 in FIG. 1) and the switching section of the electronic commutator (21 in FIG. 2) require more relaxed specifications than the power supply for the control electronics (13 in FIG. 1) or the electronic commutator logic (22 in FIG. 2). A motor is a forgiving load for a power supply. As the motor's mechanical characteristics work as a low pass filter, the motor can tolerate a relatively large ripple voltage. In fact, the ripple can be up to 100% without affecting operation. Furthermore, some applications of motors (e.g., driving a fan or turbine) can tolerate the discontinuous torque that comes with a discontinuous supply of current.
A brushless-DC-motor-driven ventilation fan shares most of the building blocks of an airflow generator for medical applications. The main differences are:                The mechanical design of the turbine or fan itself, since a flow generator needs to produce more pressure.        The ventilation fan, normally, does not interface with flow and pressure sensors. Thus, the control electronics of a ventilation fan are simpler and should draw less current.        
Regulations like the European Standard EN 60555 and the International Standard IEC 555-2 limit the current harmonic content of mains supplied equipment. This requirement applies to both the medical application of the DC motors and the ventilation fans. Power factor correction must be taken into account for all new designs. Power factor correction can add 20 to 30% to the cost of the power supply of equipment (see Ref. 10 in Appendix C). Hence, there can be a relatively substantial saving in the cost of the equipment if the function of power factor correction is integrated with the DC motor driver for equipment working from the AC mains.